1. Field of the Invention
The invention pertains to a solar thermal power system with a collector connected to a heat sink by a solar circuit containing a heat-transfer medium.
2. Related Art
Solar power systems are called solar thermal power systems when they make use of the heat of solar radiation (solar thermal energy). The heat is made usable in architectural applications or in solar thermal power plants to generate electricity. The direct conversion of sunlight into electricity is referred to as photovoltaics, and the corresponding systems are called photovoltaic systems.
To recover heat, an absorber of a thermal collector is heated by energy of the sun. A heat-transfer medium, which absorbs the heat, flows through the collector. A pump conveys the heat-transfer medium around a solar circuit, along the course of which the heat is carried from the collector to a heat sink, especially a solar heat reservoir; the solar heat reservoir accepts the heat and stores it.
The collector is the part of the solar power system that absorbs the heat of the sun and transfers the absorbed heat to the heat-transfer medium in the solar circuit with the least possible loss.
The main difference between collectors with respect to their structural design is their configuration as either flat collectors and tubular collectors. Flat and tubular collectors differ with respect to the insulation of the absorber. The insulating effect of vacuum tube collectors is achieved by a vacuum in a glass tube, which completely suppresses the transport of heat by convection.
Flat collectors use conventional insulating materials such as mineral wool or polyurethane foam. This insulation is less efficient than a vacuum, and therefore larger collector surface areas are required to achieve comparable performance values. Today's high-power flat collectors work with a copper absorber. Because flat collectors are considerably less expensive, however, and therefore are usually more economical than vacuum-tube collectors, this type of design is used almost exclusively in the residential area.
Solar heat reservoirs differ from conventional process water tanks primarily with respect to their very heavy insulation; the tall, slender shape of the water tank allows for the formation of layers of different temperature (hot water at the top, cool water at the bottom) and a heat-transfer device is installed at a low point for transferring the heat received from the solar circuit.
When a temperature in the collector rises above a temperature at the solar heat exchanger by a predetermined temperature difference, the pump is started by a control unit, and the heat is transported to the solar heat reservoir. When the temperature difference between collector and reservoir falls below the limit value, the system is stopped.
Standard commercial collectors can convert 60-70% of the solar energy arriving at the collector surface to usable heat. In these collectors, a mixture of water and polypropylene glycol, typically in a ratio of 60:40, is used as the heat-transfer medium. Through the addition of 40% propylene glycol, frost protection down to −23° C. is achieved; below that temperature, freezing will occur but without frost shattering. In addition, a boiling temperature which can be 150° C. or more, depending on the pressure, is achieved. At higher temperatures, many collectors shut down and no longer deliver energy.
Outside the heating season, solar thermal power systems are often used only to heat water. This results in a lower total efficiency of the system. When the system is idle, there is the danger of overheating and thus of damage to the solar thermal power system.